1. Field of the Invention
This invention relates to a magnetooptic reproducing device which utilizes a magnetooptic effect to optically reproduce signals recorded on a magnetic recording medium.
2. Description of the Prior Art
A method of optically reading out magnetically recorded information by the use of a magnetooptic effect such as the magnetooptic Kerr effect or Faraday effect is known and particularly, a method comprising condensing linearly polarized light on a magnetic recording medium, passing the light reflected by or passed through the magnetic recording medium through an analyzer, converting the rotation of the plane of polarization into a variation in quantity of light and thereby reading the same has been developed. An example of the device used for such reproduction of recorded information is shown in FIG. 1 of the accompanying drawings.
The Faraday effect refers to the rotation of the plane of polarization by the interaction between the light and the magnetic field as the light passes through the magnetic recording medium, and the Kerr effect refers to the rotation of the plane of polarization as the light is reflected by the magnetic recording medium.
FIG. 1A shows an example of the device for reproducing the signals on a perpendicular magnetic recording medium by utilizing the magnetooptic Kerr effect. In this example, a signal recording system for optically recording information on the perpendicular magnetic recording medium is also shown.
In FIG. 1A, the perpendicular magnetic recording medium 1 is rotated by a motor 2.
The signal recording system is the left half of FIG. 1A, and a light source such as, for example, a semiconductor laser 3 is driven by the video signal from a signal source 4. The modulated recording light beam is collimated into a parallel beam by a collimator lens 5, passes through a polarization beam splitter 6 and a quarter wave plate 7 and forms a focus on the surface of the perpendicular magnetic recording medium with the aid of an objective lens 8. Inversion of magnetic domain is caused by the thermal energy of this point image, whereby a record pattern can be recorded. The light beam for the signal recording system requires great energy for the reason set forth above and therefore, a laser light beam is usually used as such light beam, but the laser light beam from a laser light source is linearly polarized and therefore, if an ordinary half-mirror is used instead of the polarization beam splitter 6, the quantity of light reaching the recording medium 1 will be decreased to one half and this is not preferable. Accordingly, by using a polarization beam splitter having a high transmission factor for the P-polarized light, the plane of polarization of the incident laser light beam can be arranged into the P-polarized state, whereby the loss of quantity of light by the beam splitter can be minimized and a high energy point image can be formed on the recording medium.
The quarter wave plate is for making the incident linearly polarized light into circularly polarized light and making the circularly polarized light beam reflected from the recording medium into linearly polarized light rotated by 90.degree. with respect to the incident light beam, and this reflected light beam now becomes S-polarized light incident on the polarization beam splitter 6 and is reflected at a high reflection factor and received by a four-division detector 10 through a cylindrical lens 9. The system constituted by the cylindrical lens 9 and the detector 10 is for obtaining an auto-focus signal for keeping the spacing between the objective lens 8 and the recording medium 1 constant.
The right half of FIG. 1A shows the reproducing system. A light source 11 uniformly emits light. The reproducing light beam is collimated into a parallel beam by a collimator lens 12 and is caused to form a point image on the recording medium 1 by an objective lens 17 through a polarizing plate 14, a mirror 15 and a half-mirror 16. The perpendicular magnetic recording medium 1 has a signal pattern formed thereon by the difference in direction of magnetization (upward or downward). In accordance with such directions of magnetization, the light beam incident on the recording medium 1 is reflected with the plane of polarization thereof being subjected to rotations in opposite directions by the magnetooptic Kerr effect. For example, if it is assumed that the plane of polarization of the light beam reflected by the downwardly magnetized portion is subjected to rotation of .theta.k, the plane of polarization of the light beam reflected by the upwardly magnetized portion is subjected to rotation of -.theta.k.
The polarizing plate 14 is for making the incident light beam into linearly polarized light, and the light beam reflected from the recording medium has a plane of polarization rotated by +.theta.k or -.theta.k with respect to the plane of polarization of the incident light beam. This reflected light beam is separated from the incident light beam by the half-mirror 16 and, if the axis of polarization (the direction of passage of the plane of polarization) of an analyzer 18 is disposed perpendicular to -.theta.k, the light beam passing through the analyzer 18 is limited only to the polarized component of a projection component relative to the axis of polarization, of the light beam whose plane of polarization has +.theta.k. Accordingly, the recorded pattern can be converted into a light-and-dark pattern according to the upward and downward directions of the magnetic domain of the recording medium 1.
That is, where the incident light beam is P-polarized light as shown in FIG. 1B and if the transmission of polarized light through the analyzer 6 is in a direction (Q-direction) perpendicular to the direction of polarization -.theta.k, the reflected light from the upwardly magnetized portion is intercepted by the analyzer 18 and the transmission component .DELTA. of the reflected light from the downwardly magnetized portion which is to pass through the analyzer 18 passed through the analyzer 18. By this phenomenon, the perpendicularly magnetized pattern can be detected or observed.
This passed light beam is received by a four-division detector 22 through a cylindrical lens 19 and the photoelectrically converted electrical signal is separated into a recording signal and an auto-focus signal and then processed.
If the spacing between the recording medium 1 and the objective lens 17 is changed, the distribution of the line image varies and the focused condition can be detected by detecting the balance between the outputs from the elements of the four-division detector 22.
In this reproducing system, in order that the information on the angle of optical rotation of the modulated light beam emitted from the recording medium may not be lost, the half-mirror 16 for dividing the quantity of light into 50% each independently of the polarized state of the light beam is used as a beam splitter. The transmission factor and the reflection factor of this half-mirror are respectively determined to 50% because this is a value best suited to take out a maximum quantity of modulated light beam so that this set value can be easily calculated.
In the conventional reproducing system as described above, the amplitude is reduced to 1/4 because the light beam is passed twice through the half-mirror. Further, the angle of rotation .theta.k of the plane of polarization by the Kerr effect is generally very slight, e.g., about 1.degree. or less, and the quantity of light of the modulated component by the magnetooptic effect obtained by the light passing through the analyzer 18 is very minute. Accordingly, there are the following drawbacks in the conventional magnetooptic reproducing device having the reproducing optical system as described above:
1. The quantity of light of the modulated component having information is small as compared with non-information component and therefore, it is not easy to detect the recorded pattern.
2. In order to separate said minute modulated component light from the modulated light beam, a very expensive analyzer must be disposed while being position-adjusted with high accuracy and this is not preferable in respect of cost and durability.
Also, in the magnetooptic reproducing device also having the signal recording system, as shown in FIG. 1A, it has been very much desired, in terms of the versatility and reduction of size and cost of the device, to make the signal recording system and the reproducing system into a common system, namely, a common head. However, in the reproducing system of FIG. 1A, if the recording light beam is caused to enter through the same optical path as the reproducing system light beam, in whatever direction the direction of polarization may be set, the quantity of light will be reduced to 1/2 by the half-mirror 16 and the energy utilization efficiency will be aggravated, and this is not preferable. Thus, in the conventional magnetooptical reproducing device having a signal recording optical system as well, the recording system and the reproducing system differ from each other in the presence of such a part as the analyzer and moreover, differ in the characteristic which the respective light beams require of the beam splitter, and this has prevented the recording and the reproduction from being arranged into a single system.